1. Field
This disclosure relates to radio frequency filters using surface acoustic wave (SAW) resonators, and specifically to filters for use in communications equipment.
2. Description of the Related Art
As shown in FIG. 1, a SAW resonator 100 may be formed by thin film conductor patterns formed on a surface of a substrate 150 made of a piezoelectric material such as quartz, lithium niobate, lithium tantalate, or lanthanum gallium silicate. The substrate 150 may be a single-crystal slab of the piezoelectric material, or may be a composite substrate including a thin single-crystal wafer of the piezoelectric material bonded to another material such as silicon, sapphire, or quartz. A composite substrate may be used to provide a thermal expansion coefficient different from the thermal expansion coefficient of the single-crystal piezoelectric material alone. A first transducer 110 may include a plurality of parallel conductors. A radio frequency or microwave signal applied to the first transducer 110 via an input terminal IN may generate an acoustic wave on the surface of the substrate 150. As shown in FIG. 1, the surface acoustic wave will propagate in the left-right direction. A second transducer 120 may convert the acoustic wave back into a radio frequency or microwave signal at an output terminal OUT. The conductors of the second transducer 120 may be interleaved with the conductors of the first transducer 110 as shown. In other SAW resonator configurations (not shown), the conductors forming the second transducer may be disposed on the surface of the substrate 150 adjacent to, or separated from, the conductors forming the first transducer.
The electrical coupling between the first transducer 110 and the second transducer 120 is highly frequency-dependent. The electrical coupling between the first transducer 110 and the second transducer 120 typically exhibits both a resonance (where the admittance between the first and second transducers is very high) and an anti-resonance (where the admittance between the first and second transducers is very low). The frequencies of the resonance and the anti-resonance are determined primarily by the pitch and orientation of the interdigitated conductors, the choice of substrate material, and the crystallographic orientation of the substrate material. The strength of the coupling between the first transducer 110 and the second transducer 120 depends on the length L of the transducers. Grating reflectors 130, 135 may be disposed on the substrate to confine most of the energy of the acoustic waves to the area of the substrate occupied by the first and second transducers 110, 120.
SAW resonators are used in a variety of radio frequency filters including band reject filters, band pass filters, and duplexers. A duplexer is a radio frequency filter device that allows simultaneous transmission in a first frequency band and reception in a second frequency band (different from the first frequency band) using a common antenna. Duplexers are commonly found in radio communications equipment including cellular telephones.
The characteristics of SAW resonators are sensitive to the temperature of operation. A microwave filter constructed from such resonators may degrade intolerably as the operating temperature is changed unless efforts are made to mitigate the sensitivity to temperature variations. One source of temperature dependence is expansion or contraction of the piezoelectric wafer as the temperature changes. The amount a material changes dimension with respect to temperature is called the coefficient of thermal expansion (CTE). Bonding a thin piezoelectric wafer to a thicker support substrate with a lower CTE will constrain the expansion and contraction of piezoelectric wafer as the temperature changes.
FIG. 2 is a cross-sectional view of the exemplary SAW resonator 100, previously shown in FIG. 1. The electrodes forming the first transducer 110, the second transducer 120, and the grating reflectors 130, 135 are deposited on a front surface 256 of a wafer 252 of piezoelectric material that optionally may be bonded to a backing substrate 254. The wafer 252 and the backing substrate 254, when present, collectively form a composite substrate 150. The wafer 252 may be quartz, lithium niobate, lithium tantalate, lanthanum gallium silicate, or some other piezoelectric material. The backing substrate 254 may be, for example silicon, sapphire, quartz, or some other material. Typically, but not necessarily, the backing substrate 254 may be made from a material having a lower coefficient of thermal expansion than the wafer 252. The wafer 252 and the backing substrate 254 may be directly bonded using a combination of pressure and elevated temperature. Alternatively, the wafer 252 and the backing substrate 254 may be bonded using a layer of adhesive (not shown).
An issue that may occur in SAW resonators formed on thin piezoelectric wafers (e.g. piezoelectric wafers having a thickness less than about 50 acoustic wavelengths at the resonant frequency of the SAW resonator) is that the front surface 256 of the wafer 252 and the back surface 258 of the wafer 252 form a resonant cavity indicated by the arrow 250. Acoustic waves generated by the transducers 110, 120 may reflect from the back surface 258 and resonate at particular frequencies. The reflection at the back surface 258 results from a change in acoustic wave velocity at the interface between the back surface 258 and the adjacent material, which may be air, a backing substrate, or adhesive. Although the quality factor for this resonant cavity may be low, it nonetheless affects the electrical response of SAW resonator. These spurious cavity modes are commonly referred to as plate modes.
Throughout this description, elements appearing in figures are assigned three-digit reference designators, where the most significant digit is the figure number where the element is first shown and the two least significant digits are specific to the element. An element that is not described in conjunction with a figure may be presumed to have the same characteristics and function as a previously-described element having the same reference designator.